This invention relates to optical communications and specifically to a balanced coupler for coupling multiple radiation sources and an optical amplifier and transmission system using the balanced coupler.
Wave division multiplexing (WDM) increases bandwidth in optical communications by providing for communication over several wavelengths or channels. For long haul optical communications the optical signal must be periodically amplified. Current amplification schemes include Erbium doped fiber amplifiers (EDFA) and Raman amplifiers.
To maximize WDM capacity, it is desirable that the optical bandwidth of the system be as wide as possible. Thus, a wide range of optical signal wavelengths must be amplified. At the same time, it is desirable that the different optical signal wavelengths be amplified by about the same amount by the amplifiers in the amplification system. Thus, it is desirable that the amplification gain profile of the amplification system should be both broad and relatively flat.
Raman amplification can provide a broad and relatively flat gain profile over the wavelength range used in WDM optical communications by using a plurality of different pump laser wavelengths. (See Y. Emori, xe2x80x9c100 nm bandwidth flat-gain Raman Amplifiers pumped and gain-equalized by 12-wavelength channel WDM Diode Unit,xe2x80x9d Electronic Lett., Vol. 35, no. 16, p. 1355 (1999). and F. Koch et. al., xe2x80x9cBroadband gain flattened Raman Amplifiers to extend to the third telecommunication window,xe2x80x9d OFC"" 2000, Paper FF3, (2000)). Raman amplifiers may be either distributed or discrete (See High Sensitivity 1.3 (m Optically Pre-Amplified Receiver Using Raman Amplification,xe2x80x9d Electronic Letters, vol. 32, no. 23, p. 2164 (1996)). The Raman gain material in distributed Raman amplifiers is the transmission optical fiber, while a special spooled gain fiber is typically used in discrete Raman amplifiers.
FIG. 1 is a schematic of a typical optical communication system using Raman amplifiers for periodic amplification of the optical signal. The system includes transmitter terminal 10 and receiver terminal 12. The transmitter terminal includes a number of optical communication transmitters 14a, 14b, . . . 14z respectively transmitting signals at optical communications wavelengths xcexa, xcexb, . . . xcexz.
The optical signals are multiplexed by multiplexer 16 and are amplified by a series of amplifiers A1, A2, . . . An. The signals are transmitted from the transmitter 10 to the amplifiers, between the amplifiers, and from the amplifiers to the receiver 12 via transmission optical fiber 26. For distributed Raman amplification, the optical amplifier will also include transmission optical fiber. The optical signals are then demultiplexed by demultiplexer 18 of receiver 12 to respective optical communications receivers 20a, 20b, . . . 20z. The demultiplexer 18 sends optical communications wavelengths xcexa, xcexb, . . . xcexz to respective optical communications receivers 20a, 20b, . . . 20z. 
Although FIG. 1 shows signals directed from transmitter terminal 10 to receiver terminal 12 for ease of illustration, in general the transmitter terminal 10 and receiver terminal 12 are typically transmitter/receiver terminals for bidirectional communication. In this case each of the transmitter/receiver terminals will have transmitters as well as receivers and both a multiplexer and demultiplexer.
FIG. 2 is a schematic of a typical distributed Raman optical amplifier 50 employed as one of the amplifiers in the series of amplifiers A1, A2, . . . An in the system of FIG. 1. The amplifier 50 includes optical pump assembly 51 (shown enclosed by dashed lines) and transmission fiber 64. In this amplification scheme, the pump assembly 51 includes a pump radiation source 52 that provides, for example, twelve different pump wavelengths xcex1 through xcex12. Specifically, the pump radiation source 52 comprises a plurality of pump sources, i.e., twelve lasers 56 that each emit radiation at a different wavelength of the wavelengths xcex1 through xcex12, respectively. The radiation from the individual radiation sources 56 of the pump radiation source 52 are then coupled or combined at a pump radiation coupler 54, and the coupled radiation is output at pump radiation coupler output 58.
The coupled radiation has a coupled radiation profile that is a combination of the individual radiation profiles of the radiation input into the pump radiation coupler 54. The pump radiation profile, that will be coupled with the optical signal to be amplified, is therefore the coupled radiation profile in this case. Thus, the pump radiation profile is output from output 58. The pump radiation profile from output 58 is then coupled at pump-signal combiner 60 with the optical signal 62. Optical signal 62, i.e., the data signal, propagates in the transmission optical fiber 64 in a direction opposite to the radiation, i.e., a counterpropagation direction, of the pump radiation profile. The optical signal is amplified along transmission optical fiber 62.
It would be desirable to provide an optical coupler system that could provide substantially the same optical output power at each output of the coupler.
According to one embodiment of the invention there is provided an optical coupler system. The optical coupler system comprises: a first optical coupler having at least a first and a second input and a first and a second output; and a second optical coupler having at least a first and a second input and a first and a second output. The first and second outputs of the first optical coupler are connected to the first and second inputs, respectively, of the second optical coupler via first and second optical links, and the radiation that is input to the first input of the first optical coupler is coupled to both the first and second optical links to travel over first and second paths as first path radiation and second path radiation. At the second coupler the second path radiation is incoherently combined with the first path radiation for output on the first output of the second coupler.
According to another embodiment of the invention there is provided an optical coupler system. The optical coupler system comprises: a first optical coupler having at least a first and a second input and a first and a second output; and a second optical coupler having at least a first and a second input and a first and a second output. The first and second outputs of the first optical coupler are connected to the first and second inputs, respectively, of the second optical coupler via first and second optical links. The first and second links provide different optical paths between said first and second optical couplers such that portions of radiation energy that is input to said first input of said first optical coupler are combined incoherently at said first output of said second optical coupler.
According to another embodiment of the invention there is provided an optical coupler. The optical coupler system comprises: a series of N couplers optically connected in series, where N is an integer greater than 1, the couplers in the series numbered i=1 to i=N, each ith coupler having at least first and second inputs and at least first and second ouputs; and a series of Nxe2x88x921 groups of optical links, the series of groups numbered j=1 to j=Nxe2x88x921, wherein each optical,link of the jth group of optical links optically connects a respective output of the ith coupler to a respective input of the (i+1)th coupler when i=j. The optical links provide different optical paths between said first and Nth optical couplers such that portions of radiation energy that is input to said first input of said first optical coupler are combined incoherently at said first output of said Nth optical coupler.
According to another embodiment of the invention there is provided a method of coupling radiation. The method comprises: inputting radiation from a first radiation source of a plurality of radiation sources into a first input of a first optical coupler having a plurality of inputs, wherein the first optical coupler is coupled to a second optical coupler via a plurality of optical links, the second coupler having a plurality of outputs including a first output; propagating portions of the radiation along different respective optical paths between the first input of the first optical coupler and the first output of the second optical coupler; coupling the portions of the radiation at the second coupler; and wherein the optical links provide different optical paths between said first and second optical couplers such that the portions of radiation energy that is input to said first input of said first optical coupler are combined incoherently at the second coupler.
According to another embodiment of the invention there is provided an optical pump assembly. The optical pump assembly comprises: a plurality of pump radiation sources; and an optical coupler system. The optical coupler system comprises: a first optical coupler having at least a first and a second input and a first and a second output, the first and second input adapted for receiving radiation from respective radiation sources of the plurality of pump radiation sources; and a second optical coupler having at least a first and a second input and a first and a second output. The first and second outputs of said first optical coupler are connected to said first and second inputs, respectively, of said second optical coupler via first and second optical links. The first and second links provide different optical paths between said first and second optical couplers such that portions of radiation energy that is input to said first input of said first optical coupler are combined incoherently at said first output of said second optical coupler.